1. Field of the Invention
The present invention relates to a method and apparatus for valving the flow of fluid in a fluid applicator or coating systems and the like. More specifically, the invention is in one important application concerned with mechanical or electromechanical valving devices for controlling the depositing from nozzles upon moving surfaces of predetermined amounts of adhesive or other coating fluids.
2. Description of the Prior Art
In the art of fluid deposition on moving surfaces it is well known to use a two-way poppet valve with a single fluid supply input to the valve assembly. One such is described, for example, in the "Extruder Valve" 1977 bulletin of Acumeter Laboratories, the assignee of the present invention. A pressure relief valve, located at the source fluid pump, relieves any excess pressure when the two-way poppet valve is closed. Upon opening the poppet valve to apply fluid to the moving surface, the internal pressure discharges, causing a sudden surge with subsequent dropoff of pressure and fluid displacement. This effect in turn causes a heavy deposit and then a light deposit on the surface being coated.
Additionally, in traditional two-way poppet valve assemblies, the required minimum size of the fluid capacitance chamber causes retention of excess fluid. After the poppet valve is closed, therefore, a substantial amount of fluid (termed after-drool) is deposited irregularly on the surface.
Such valve and associated switch assemblies, moreover, require a long stroke of the poppet valve piston with a subsequent long on/off cycle time which traditionally can be reduced only to about 40 milliseconds. The speed of the moving surface must thus be reduced below desired high speeds if limited portions of intermittent coatings on the surface are desired, resulting in relatively inefficient operation.
Finally, some fluid deposition systems, such as nozzle extruders of the type described in a 1982 bulletin entitled "Wide Band Extrusion Nozzles" of said Acumeter Laboratories, cause extreme shear along the longitudinal sides of the piston. Many adhesives and other fluids will not accept excessive shear, with the result that the extruder valve piston will not reciprocate.
The present invention, on the other hand, provides a novel three-way poppet valve assembly that unlike the above-described valves will permit precise, constant thickness patterns of fluid coating while causing only a minimum of after drool. Additionally, the present invention provides an apparatus with a piston having a short stroke to allow more rapid on/off cycle times, and of a design so as to reduce the created fluid shear at critical points, allowing the piston to reciprocate.